Prostate cancer (PCa) is an age-related disease. Both activation of oncogenes and loss of tumor suppressor genes have been implicated in PCa development. Our recent work suggests that 15-1ipoxygenase 2 (15-LOX-2), an enzyme that metabolizes membrane phospholipid arachidonic acid (AA), is a functional prostate tumor suppressor. The molecule is abundantly expressed in the adult prostate gland but lost in PCa. 15-LOX-2 re-expression in PCa cells inhibits their proliferation in vitro and tumor development in vivo. How 15-LOX-2 suppresses PCa development is incompletely understood. Newly emerged data from our lab suggests that 15-LOX-2 is a prostate senescence gene. In cultured normal human prostate (NHP) epithelial cells, the mRNA and protein levels of 15-LOX-2 and its splice variants accumulate, preceding the replicative cell senescence. 15-LOX-2 expression in NHP cells in vivo is also correlated with age. Gain-of-function experiments reveal that restoration of 15-LOX-2 or its splice variant expression in PCa cells induces a senescence-like phenotype. Targeted expression of 15-LOX-2 or 15-LOX-2sv-b, a splice variant that lacks AA-metabolizing activity, in transgenic animals causes epithelial alterations associated with oxidative stress and senescence. Loss-of-function experiments using small interfering RNA (siRNA) to transiently knock down 15-LOX-2 expression promotes cell-cycle progression and youthful characteristics in NHP cells. Mechanistic studies suggest that the 15-LOX-2-induced NHP cell senescence is not related to its AA-metabolizing enzymatic activity, but rather to p53- and pl6tpRb-related cell-cycle arrest caused by increased oxidative stress. Based on these findings, we hypothesize that 15-LOX-2 is a prostate senescence gene with tumor suppressive functions and loss of 15-LOX-2 expression may allow cells to escape replicative senescence and thus contributes to PCa development. We further hypothesize that 15-LOX-2-mediated NHP cell senescence may involve cell-cycle arrest caused by increased oxidative stress. We propose the following three Specific Aims to test our hypotheses: 1) To study the role of 15-LOX-2 splice variants in NItP cell senescence: Correlation of their expression with prostate age in vivo; 2) To establish the cause and effect relationship: Studies using transgenic animal models and stable RNAi knockdown; and 3) To dissect the molecular mechanisms of NHP cell senescence mediated by 15-LOX-2 and its splice variants. These aims will be accomplished by a combination of cell biological, biochemical, and molecular methods, as well as utilizing transgenic animals. Our long-term goal is to utilize the data obtained from this project to develop 15-LOX-2-targeted anti-PCa therapeutics that trigger cell-cycle arrest and senescence.